Silica-titania glass, for example, ULE® glass (Corning Incorporated) is the material of choice for mirror substrates for use in extreme ultraviolet lithography (“EUV”) tools due to its very low coefficient of thermal expansion (“CTE”). The substrates are specified within a very narrow range for the value of the average CTE zero crossover temperature (“Tzc”), which is controlled by glass composition and by the thermal history of the glass. Qualification of a glass to ensure that it fulfills specification requirements for Tzc involves measurements of CTE using an ultrasonic method. While the indirect ultrasonic method has been highly successful to date, it does have some shortcomings. For example:                1. It relies on the material having a well-defined thermal history. Measuring a material with different thermal history requires the calibration to be corrected for the specific thermal history of the material with the different thermal history.        2. There is potential for uncontrolled factors, for example, the OH content, affecting the calibration and going unnoticed, which would introduce errors in the Tzc calculated for the part.        3. Efforts to correlate the technique to absolute dilatometry show a residual error in the order of 1 to 2° C. in the crossover temperature calculated for the parts.        4. Due to its indirect nature, and its reliance on an empirical calibration, customers are uncomfortable relying on its results for qualifying material when requirements for Tzc accuracy are in the order of a few degrees C.        
On the other hand, the value of Tzc can be ascertained by measuring a sample of glass in an absolute dilatometer, for example, a Fabry-Perot interferometer. While absolute dilatometry is a well established technique, it is not suitable for controlling glass in a production environment because:                1. It requires carefully finished samples, which are expensive and take a long time to manufacture (4 to 8 weeks).        2. It requires expensive specialized equipment and personnel.        3. It is potentially affected by subtle and hard to quantify effects such as the temperature dependence of reflection coatings, and the quality of optically contacted bonds.        4. Due to the relatively large size of the needed samples, it is sometimes hard to select a sample that truly represents the material used to make a part.        5. It is very slow, typically taking weeks to measure a sample.        
The photoelastic sandwich seal technique can be used to measure the difference in CTE between samples of two materials using much simpler and faster equipment than is required and used for absolute dilatometry. However, there are some shortcomings to the photoelastic sandwich seal technique, for example:                1. It also requires relatively expensive and carefully made samples, with a long lead time.        2. It measures differences in CTE between two materials, and does not directly measure the absolute Tzc. Establishing absolute Tzc requires correlation to a reference technique.For these reasons the photoelastic sandwich seal technique is not well suited for direct Tzc characterization in a production environment.        
Thus, in view of the deficiencies of the known methods for measuring Tzc, there is a need for a technique that allows quick and inexpensive measurement of the absolute Tzc of a small sample of ULE® glass without the need for expensive equipment or samples that have high cost and take a long time to manufacture. In addition, such replacement method and associated equipment should be usable in production to provide an absolute reference for interferometry, which would allow this higher resolution technique to replace highly labor intensive, lower spatial resolution ultrasonic velocity measurements that are presently being used in the industry.